Over-Whisking Egg Whites
Over-whisked egg whites are easy to recognise – they become a lumpy, curdled-looking mess and ooze excess liquid. Yep, not pretty, and unfortunately once they reach this stage there is no way to fix them. Over-whisking egg whites are common, but if you know what is going on they are easy to avoid.
When you start to whisk, the protein bonds in the egg whites are pulled apart and uncoil, breaking up the original thick viscose liquid structure. As whisking continues, the proteins then loosely re-link and re-bond around tiny air bubbles and moisture to create a foam. If you continue to whisk, these protein bonds will eventually tighten so much that they are no longer able to hold moisture, forcing the moisture out, breaking down the foam and turning it into a curdled mess.
The best way to avoid this is to whisk your egg whites by hand with a balloon whisk or, if using an electric mixture, on medium (or at least no higher than medium-high). Keep a really close eye on it, stopping frequently and checking when you are close to the point you want to reach (usually soft or firm peaks) and only whisking for brief portions of time after this. Whisking by hand, because it is naturally slower, will give you far more control over your foam than whisking with an electric mixer and hence there will be less chance of over-whisking.